1. Field of the Invention
This invention relates to a method for producing acrylic acid by distilling an acrylic acid-containing solution without using an azeotropic solvent, obtaining crude acrylic acid as a column bottom stream and/or a column side stream of a distillation column, and then crystallizing the crude acrylic acid.
2. Description of the Related Art
Commercial production of acrylic acid generally resorts to the method of propylene oxidation which consists in subjecting propylene and/or acrolein to catalytic gas phase oxidation. When acrylic acid is produced by this method of propylene oxidation, the step of propylene oxidation gives rise to such impurities as water, acids like propionic acid, acetic acid, and maleic acid, and aldehydes like acetone, acrolein, furfural, and formaldehyde in the form of by-products. The gas containing these by-products is absorbed as an acrylic acid-containing solution generally via contact with an absorbent and obtained the solution is subsequently separated the absorbent by distillation etc.
One known method, for example, produces acrylic acid of high purity by absorbing an acrylic acid-containing gas resulting from catalytic gas phase oxidation with a high boiling solvent, distilling the acrylic acid-containing solution thereby separating it into the solvent and crude acrylic acid, and subsequently subjecting the crude acrylic acid to a process of crystallization (JP-A-1997-227445). This method comprises carrying out a step of absorption of an acrylic acid-containing gas after the gas has been cooled with a venturi, carrying out a step of removing a low boiling compound subsequent to the step of absorption, thereafter separating the acrylic acid-containing solution in a distillation column into a high boiling substance and a low boiling substance, withdrawing crude acrylic acid as a liquid via the intermediary stage of the column, and purifying the crude acrylic acid by a step of crystallization.
When an aqueous solution can be used as an absorbent for acrylic acid in the place of an expensive high boiling solvent, the use of this aqueous solution proves economical. Particularly, when this aqueous solution is capable of absorbing the acrylic acid-containing solution in high concentration, the use of the aqueous solution is efficient in respect that it results in decreasing the amount of treatment at the subsequent steps of purification. A method has been disclosed which comprises introducing an acrylic acid-containing gas into an absorption column, absorbing with aqueous solution containing acetic acid thereby effecting absorption of acrylic acid therein, and producing as the bottom liquid of the absorption column an acrylic acid-containing solution composed of 50–80 wt % of acrylic acid, 2–5 wt % of acetic acid, and the balance of water (JP-A-1993-246941).
This method obtains purified acrylic acid by subjecting the acrylic acid-containing solution to azeotropic dehydration using a mixed solution of two or more azeotropic solvents and subsequently passing the product of dehydration through such steps as the removal of high boiling substance.
Still another method has been disclosed which, in the absorption with water of an acrylic acid-containing solution resulting from a reaction of catalytic gas phase oxidation, comprises supplying the recovery water emanating from the step of azeotropic dehydration to the absorption column, supplying the resultant acrylic acid-containing solution to a stripping column, and obtaining an acrylic acid solution composed of 70.9 wt. % of acrylic acid, 25.6 wt. % of water, and 2.0 wt. % of acetic acid via the bottom of the stripping column (JP-A-2001-199931). This method obtains purified acrylic acid by performing azeotropic dehydration of the acrylic acid-containing solution and subsequently subjecting the product of dehydration to the step of crystallization.
For the purpose of obtaining crude acrylic acid substantially destitute of water by treating an aqueous acrylic acid solution by distillation, it is common to perform azeotropic distillation using an azeotropic solvent. The azeotropic distillation necessitates a step of solvent separation in order to reuse the azeotropic solvent. In JP-A-1993-246941 and JP-A-2001-199931 which disclose inventions involving use of an aqueous type absorbing solvent mentioned above, azeotropic dehydration using an azeotropic solvent is described. Thus, the development of a method which produces acrylic acid without a step of azeotropic distillation has been yearned for enthusiastically.
An attempt to obtain crude acrylic acid substantially destitute of water without performing azeotropic distillation results in causing acrylic acid to distill in a considerable amount from the top of the distillation column in consequence of azeotropy between acrylic acid and water, increasing the amount of acrylic acid expelled out of the system, and causing degradation of yield. Under these existing circumstances, the development of a method for producing acrylic acid in a high yield has been yearned for.